Conventionally, there has been known a light-emitting type display device having a display panel in which organic electroluminescence devices (hereinafter referred to as “organic EL devices”), inorganic electroluminescence elements (hereinafter referred to as “inorganic EL devices”) or self-luminous light emitting devices (optical elements) such as light-emitting diodes (LEDs) and the like are arranged in a matrix form.
Particularly, the light-emitting type display device using an active matrix drive system has higher display response speed than the liquid crystal display device that has recently sprung into wide use, no dependence on an angle of field, and is capable of providing high luminance and contrast, high definition of quality of display image, a reduction of power consumption, and the like. The light-emitting type display device has an extremely advantageous characteristic in which no backlight is required unlike the liquid crystal display device to allow the device to be much thinner and lighter.
Here, in the aforementioned display device having various kinds of light-emitting devices, drive control mechanisms for providing controlling light-emission control to light-emitting devices and control methods have been variously proposed. For example, there has been known a drive circuit (hereinafter referred to “pixel drive circuit” for the sake of convenience) having a plurality of switching devices such as thin-film transistors for providing the light-emission control to light-emitting devices for each of display pixels that forms the display panel in addition to the aforementioned light-emitting devices.
The following will explain a circuit diagram that is applied to display pixels of the display device having organic EL devices, which use organic compounds that have recently studied and developed actively toward practical use as light-emitting materials, among the aforementioned various kinds of light-emitting devices, with reference to the drawings.
FIGS. 11A and 11B are circuit diagrams each illustrating an example of the structure of the display pixel of the prior art in the light-emitting device type display device having organic EL devices.
For example, as shown in FIG. 11A, in the vicinity of each intersection point of plural scan lines SL and a data line DL that are arrayed in a matrix form on the display panel, the display pixel of the prior art is structured to have a pixel drive circuit DP1, which includes a thin-film transistor Tr 11 where a gate terminal is connected to the scan line SL, a source terminal and a drain terminal are connected to the data line DL and a node 11, respectively, and a thin-film transistor Tr 12 where a gate terminal is connected to the node N11 and a source terminal is connected to a power line VL, respectively, and an organic EL device (light emitting device) OEL where an anode terminal is connected to the drain terminal of the thin-film transistor Tr12 of the pixel drive circuit DP1 and a cathode terminal is connected to a ground potential. In this case, in FIG. 11A, C11 denotes a parasitic capacitance that is formed between the gate and source of the thin-film transistor Tr12.
In other words, the pixel drive circuit DP1 illustrated in FIG. 11A is structured such that two transistors of thin-film transistors Tr11 and Tr12 are ON-OFF controlled to provide light-emission control to the organic EL device OEL as shown in below.
In the pixel drive circuit DP1 having such a structure, when a high-level scan signal is applied to the scan line SL to set the display pixel to a selection state by a scan driver (omitted in the figure), the thin-film transistors Tr11 is turned on, thereby a signal voltage (gray-scale voltage) applied to the data line DL by a data driver (omitted in the figure) is applied to the gate terminal of the thin-film transistor Tr12 via the thin-film transistor Tr11 in accordance with display data (image signal). As a result, the thin-film transistor Tr12 turns on in an electrically continuous state according to the above signal voltage, so that a predetermined drive current flows from the power line VL via the thin-film transistor Tr12 and the organic EL device OEL emits with a luminance gray-scale according to display data.
Next, when a low level scan signal is applied to the scan line SL to set the display pixel to a non-selection state, the thin-film transistor Tr11 is turned off, thereby the data line DL and the pixel drive circuit DP1 is electrically disconnected. As a result, the voltage applied to the gate terminal of the thin-film transistor Tr12 is held by the parasitic capacitance C11 and the thin-film transistor Tr12 is maintained in an ON state, so that a predetermined drive current flows into the organic EL device OEL and the light-emitting operation is continued. This light-emitting operation is controlled to be continued for, e.g., one frame period until the signal current is written to the each display pixel according to next display data.
Such the driving method is called as a voltage drive system for the reason that the drive current to flow to the light-emitting device is controlled by adjusting the voltage to be applied to each display pixel to operate light-emission with a predetermined luminance gray-scale.
Moreover, for example, as shown in FIG. 11B, in the vicinity of each intersection point of first and second scan lines SL1 and SL2, which are arrayed in parallel to each other, and data lines D, the display pixel of the prior art as another example is structured to have a pixel drive circuit DP2, which includes a thin-film transistor Tr21 where a gate terminal is connected to the first scan line SL1, and a source terminal and a drain terminal are connected to the data line DL and a node N21, respectively, a thin-film transistor Tr22 where a gate terminal is connected to the second scan line SL2 and a source terminal and a drain terminal are connected to nodes N21 and N22, respectively, a thin-film transistor Tr23 where a gate terminal is connected to the node N22 and a source terminal is connected to the power line VL and a drain terminal is connected to the node N21, respectively, a thin-film transistor Tr24 where a gate terminal is connected to the node N22 and a source terminal is connected to the power line VL, respectively, and an organic EL device (light emitting device) OEL where an anode terminal is connected to the drain terminal of the thin-film transistor Tr24 of the pixel drive circuit DP2 and a cathode terminal is connected to a ground potential.
Here, in FIG. 11B, the thin-film transistor Tr21 is formed of a n-channel type MOS transistor (NMOS), and each of the thin-film transistors Tr22 to Tr24 is formed of a p-channel type MOS transistor (PMOS). C21 denotes a parasitic capacitance that is formed between the gate and source of each of the thin-film transistors Tr23 and Tr24 (between node N 22 and power line VL). In other words, the pixel drive circuit DP2 illustrated in FIG. 11B is structured such that four transistors of thin-film transistors Tr21 to Tr24 are ON-OFF controlled to provide light-emission control to the organic EL device OEL as shown in below.
In the pixel drive circuit having such a structure, when a low-level scan signal and a high-level scan signal are applied to the scan lines SL1 and SL2, respectively, to set the display pixel to a selection state by a scan driver (omitted in the figure), the thin-film transistors Tr21 and Tr22 are turned on, thereby a signal current (gray-scale current) supplied to the data line DL by a data driver (omitted in the figure) is fetched to the node N22 via the thin-film transistors Tr21 and Tr22 in accordance with display data, and the signal current level is converted to a voltage level by the thin-film transistor Tr23, so that a predetermined voltage occurs between the gate and source (writing operation).
After that, for example, when a low-level scan signal is applied to the scan line SL2, the thin-film transistor Tr22 is turned off, thereby the voltage occurred between the gate and source of the thin-film transistor Tr23 is held by the parasitic capacitance C21. Next, when a high-level scan signal is applied to the scan line SL1, the thin-film transistor Tr21 is turned off, thereby the data line DL and the pixel drive circuit DP2 are electrically disconnected. As a result, the thin-film transistor Tr24 is turned on, so that a predetermined drive current flows from the power line VL via the thin-film transistor Tr24 and the organic EL device OEL emits with a luminance gray-scale according to display data (light-emitting operation).
Here, a drive current to be supplied to the organic EL deice OEL via the thin-film transistor Tr24 is controlled to reach a current value that is based on the luminance gray-scale of display data, and this light-emitting operation is controlled to be continued for, e.g., one frame period until the signal current is written to the each display pixel according to next display data.
Such the driving method is called as a current designation system for the reason that the current where the current value is designated to each display pixel according to display data is supplied and the drive current to flow to the organic EL device is controlled based on the voltage held according to the current value to perform a light-emitting operation with a predetermined luminance gray-scale.
However, the display device with the aforementioned various kinds of pixel drive circuits in the display pixel has the following problems.
Namely, the pixel drive circuit using the voltage drive system as illustrated in FIG. 11A has the problem in that when device characteristics of two thin-film transistors Tr 11 and Tr12 such as a channel resistance, and the like are changed by ambient temperature, variation with the passage of time, and the like, this exerts an influence upon the drive current supplied to the light-emitting devices to make it difficult to realize a predetermined light-emitting characteristic stably for a long time.
Moreover, there is a problem in that when each of the display pixels that forms the display panel is made finer to improve high definition of the display image quality, a variation in the operation characteristic such as source-drain current of each of the thin-film transistors Tr11 and Tr12 that forms the pixel drive circuit increases, so that appropriate gray-scale control cannot be performed and a variation in the display characteristic of each display pixel occurs, causing deterioration in the image quality.
Further, in the pixel drive circuit illustrated in FIG. 11A, it is necessary to use the PMOS transistor as the thin-film transistor Tr12 such that the source terminal of thin-film transistor Tr12, which supplies the drive current to the light-emitting devices, is connected to the power supply line VL and the cathode terminal of the light-emitting device is connected to the ground potential in view of the circuit structure to continue the light-emitting operation in a non-selection state. In this case, when amorphous silicon is used, the PMOS transistor with the sufficient operation characteristic and function cannot be formed. For this reason, the manufacturing techniques for polysilicon and monocrystal silicon must be used in the case of the structure in which the PMOS transistor is mixed in the light-emitting drive circuit. However, the manufacturing techniques using polysilicon and monocrystal silicon are complicated in the manufacturing process and expensive in the manufacturing cost as compared with the manufacturing techniques using amorphous silicon. This causes a problem in an increase in the manufacturing cost of the display device having the light-emitting drive circuits.
Furthermore, in the pixel drive circuit using the current designation system as illustrated in FIG. 11B, the thin-film transistor Tr23, which converts the current level of the signal current supplied to each display pixel according to display data to the voltage level, and the thin-film transistor Tr24, which supplies the drive current with a predetermined current value, are provided, the influence caused by variations in the operation characteristic of each thin-film transistor can be suppressed to a certain extent by setting the signal current to be supplied to the light-emitting devices.
However, in the pixel drive circuit using the aforementioned current designation system, for writing the signal current, which is based on display data with relatively low luminance gray-scale, onto each display pixel, it is necessary to supply the signal current with a small value corresponding to the luminance gray-scale of display data. However, the operation for writing display data onto each display pixel is equivalent to the fact that the data line is charged up to a predetermined voltage. Particularly, when the wire length of the data line is designed to be longer because of the increase in the size of the display panel, there occurs a problem in that the smaller the current value of the signal current becomes, the more time required for a writing operation to the display pixel increases. As a result, when the number of scan lines is increased with high definition of the display panel and the selection time of the scan line is set to be short, the writing operation to the display pixel becomes insufficient at the low gray-scale time, making it difficult to obtain a good quality of the display image.
In contrast to this, for example, the pixel drive circuit as illustrated in FIG. 11B is structured such that the thin-film transistors Tr23 and Tr24 form a current mirror circuit structure and the current to be supplied to the display pixel becomes small with respect to the signal current to be supplied to the data line. As a result, even if the signal current with a relatively small current value is written to each display pixel at the low gray-scale time, the current value of the current to be supplied to the data line can be made relatively large, and time required for a writing operation to the display pixel is reduced to make it possible to improve the quality of display image.
However, in the pixel drive circuit having such the structure, the value of the current to be supplied to the data line is proportional to the drive current to be supplied to the light-emitting devices and becomes a value with predetermined ratio times of the drive current. For this reason, when the current ratio is set to such a value that the writing operation can be sufficiently performed even at the minimum gray-scale time, the value of the signal current to be supplied to the data line becomes an excessive value at an upper gray-scale time, causing a problem in that power consumption for the display device is increased.